1. Field of the Invention
The present invention relates to an error recovery apparatus of a digital broadcasting receiver, and more particularly, to an error recovery apparatus of a digital broadcasting receiver to compensate a sampling timing offset and a carrier frequency offset of a corresponding broadcasting signal when a digital broadcasting signal is restored in which a vestigial sideband modulation method is applied.
2. Description of the Related Art
To transmit a broadcasting signal to realize digital broadcasting, a VSB (vestigial sideband) modulation method and a COFDM (coded orthogonal frequency division multiplexing) modulation method are provided. The VSB modulation method transmits the broadcasting signal to a single carrier. The COFDM modulation method transmits the broadcasting signal through a multi-transmission-route by multiple-dividing the broadcasting signal. At present, the VSB modulation method is used in Korea and the USA, and the COFDM modulation method is a European style digital broadcasting transmission method.
FIG. 1 is a block diagram showing a conventional error recovery apparatus of a digital broadcasting receiver receiving the broadcasting signal transmitted using the VSB (vestigial sideband) modulation method. The error recovery apparatus has an ADC (analog to digital converter) 11, an I/Q (in-phase/quadrature) splitter 13, an STR (symbol timing recovery) 17, an interpolator 15, a DFPLL (digital frequency phase locked loop) 21, an MF (matched filter) 23, and an equalizer 25.
The ADC 11 converts the transmitted broadcasting signal having an analog format into a digital format by digitally sampling the transmitted broadcasting signal. The I/Q splitter 13 shifts the broadcasting signal of the digital format to a baseband, and splits the broadcasting signal into an in-phase channel signal and a quadrature channel signal.
The SRT 17 detects timing error with respect to a symbol corresponding to the broadcasting signal generated when the broadcasting signal is being sampled at the ADC 11. The interpolator 15 restores a timing of the corresponding symbol based on a timing error value detected at the STR 17. Moreover, the interpolator 15 calculates clock information corresponding to the restored timing. The DFPLL 21 corrects a frequency offset with respect to the broadcasting signal, in other words, the DFPLL 21 corrects a phase of the broadcasting signal and distortion of a carrier signal through a pilot signal when transmitting the broadcasting signal and the clock information calculated at the interpolator 15. At this time, the frequency offset and broadcasting signal that compensate the distortion of the carrier signal are multiplied with a broadcasting signal output from the interpolator 15 through a multiplier 19 and then output to the MF 23.
The MF 23 filters the broadcasting signal output from the multiplier 19 in order to maximize a ratio of noise to signal in relation to the broadcasting signal output. The equalizer 25 corrects the error generated in the transmission of the broadcasting signal output from a transmission terminal.
However, the broadcasting signal transmitted using the VSB modulation method from the transmission terminal is transmitted through a broadcasting channel formed by air, where a transmission channel of a sub-path can be formed besides a main-path due to reflection by obstacles. When the broadcasting signal transmitted through the multi-path channel is restored through the conventional error recovery apparatus of the digital broadcasting signal receiver, an offset is generated in relation to the broadcasting signal transmitted though the sub-path compared with the broadcasting signal transmitted through the main-path. The broadcasting signal having the above phase offset is input into the equalizer 25, and the broadcasting signal having the phase offset becomes a reason of deteriorating an operation efficiency of the equalizer 25 and the entire apparatus. Further, the phase offset of the broadcasting signal transmitted through the sub-path has different values based on the transmission path of the broadcasting signal.
When the multi-path channel is a single ghost environment having an amplitude ‘a’ and a phase ‘θ’, a response (h(t)) of the channel can be expressed as the following mathematical expression 1:h(t)=δ(t)+αejθ·δ(t−τ)
Therefore, the response value (h(t)) has a different value when a receiving time of the broadcasting signal is different based on the channel in relation to the multi-path channel.
A pilot tone (P(t)) showing a level of a pilot signal of the broadcasting signal transmitted through the multi-path channel has a phase offset expressed in the following mathematical expression 2:
      P    ⁢          (      t      )        =            tan              -        1              ⁡          [                                    -            α                    ·                      sin            ⁢                          (                                                2                  ⁢                  π                  ⁢                                                                          ⁢                  f                  ⁢                                                                          ⁢                  τ                                +                θ                            )                                                1          +                      α            ·                          cos              ⁢                              (                                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                    f                    ⁢                                                                                  ⁢                    τ                                    +                  θ                                )                                                        ]      
where, ‘f’ is a difference of a center frequency (fm) for the broadcasting signal and a frequency (fp) of the pilot tone.
According to the mathematical expression 2, a maximum phase offset generated in relation to the change of the phase θ is given as sin−1α. Yet, the interpolator 15 performs the timing recovery so that the phase of the broadcasting signal transmitted through the main path and that of the pilot tone transmitted through the sub path can be identical. Therefore, the interpolator 15 performs the timing recovery to reduce the offset, when the offset is generated in relation to the broadcasting signal. At this time, the carrier timing recovery falls to a half of a phase value of the broadcasting signal transmitted through the main path and the broadcasting signal transmitted through the sub path.
Therefore, in the conventional error recovery apparatus of the digital broadcasting receiver, the broadcasting signal is applied to the equalizer 25 without full recovery of the phase offset of the broadcasting signal according to the multi-path channel, which deteriorates an efficiency of the equalizer 25.
Therefore, in the conventional error recovery apparatus of the digital broadcasting receiver, when the broadcasting signal intervenes after being transmitted through more than one channel besides the main path that is the multi-path channel, the phase error is bound to be generated, even through the carrier and the timing recovery in relation to the broadcasting signal transmitted through the main path is done. As the broadcasting signal including the generated phase error is applied, the efficiency of the equalizer 25 is deteriorated.